The earliest form of a book. The Latin term, originally used for hinged waxed wooden writing tablets, came to designate a collection of sheets of papyrus or parchment stitched up the middle & written on both sides. Thus each sheet produced 4 written pages. This compact format was far more economical & efficient than scrolls written only on one side that had to be carefully unrolled & re-rolled with each use. Christians gave momentum to the development of the codex. By the 2nd c. CE Christian writings that had been originally composed on scrolls were generally copied into single book codices. By the 3rd c. CE, codices containing two or more works were in circulation. But no extensive collection of scriptures in one volume was possible before the 4th c. CE when a system of binding several quires of folios together was perfected. The most important codices from the century in which Christianity became legal are the oldest complete single volume NTs (known as Sinaiticus & Vaticanus) and the 12 volume gnostic library discovered at Nag Hammadi, Egypt.

[For more details see R. W. Funk, Honest to Jesus (San Francisco: HarperCollins, 1996), pp. 107-110].

Other On-line Resources:

• Codex - images & facts about ancient codices posted by Timothy W. Seid (Interpreting Ancient Manuscripts).

• Codex - brief history of the word from the American Heritage Dictionary 
  4th ed. (posted by Houghton Mifflin Harcourt).

• Codex - descriptive definition from Bookbinding & the Conservation of Books by Matt Roberts & Don Etherington (Conservation on Line).

• Adoption of the Codex Book - essay by professional bookbinder Gary Frost (American Institute for Conservation of Historic and Artistic Works).